The subject invention relates to optical metrology equipment for measuring critical dimensions and feature profiles of periodic structures on semiconductor wafers. The invention is implemented using data obtained from simultaneous multiple angle of incidence measurements as an input to analytical software designed to evaluate surface features via a specular scatterometry approach.
There is considerable interest in the semiconductor industry in evaluating small features of periodic structures on the surface of a sample. In current high density semiconductor chips, line widths or feature sizes are as small as 0.1 microns. These feature sizes are too small to be measured directly with conventional optical approaches. This is so because the line widths are smaller than the probe beam spot size which can be achieved with most focusing systems.
This problem is illustrated in FIG. 1 which shows a wafer 10 having formed thereon a number of conductive lines 12. A probe beam 14 is shown focused by a lens 20 onto the sample at a spot 16. The reflected beam is measured by a photodetector 18. As can be seen, spot 16 overlaps multiple lines 12 and therefore cannot be used to measure distances between lines or the thickness of the lines themselves.
To overcome this problem, sophisticated software programs have been developed which analyze the reflected probe beam in terms of a scattering model. More specifically, it is understood that critical dimensions or feature profiles on the surface of the wafer will cause some level of scattering of the reflected probe beam light. If this scattering pattern is analyzed, information about the critical dimensions can be derived. This approach has been called specular scatterometry. The algorithms use various forms of modeling approaches including treating the lines as an optical grating. These algorithms attempt to determine the geometry of the periodic structure.
FIG. 2 schematically illustrates the geometry of one type of periodic structure 24. This periodic structure can be analyzed in terms of the width W between the features and the depth D of the grooves. In addition, the shape or profile P of the side walls of the features can also be analyzed by the current algorithms operating on the analytical data.
To date, these analytical programs have been used with data taken from conventional spectroscopic reflectometry or spectroscopic ellipsometry devices. In addition, some efforts have been made to extend this approach to analyzing data from simultaneous multiple angle of incidence systems. In these systems, the spot size is relatively small, but still larger than the individual features of the periodic structure. Paradoxically, where the features are only slightly smaller than the spot size, analysis through scatterometry is difficult since not enough of the repeating structure is covered by the spot. Accordingly, it would be desirable to modify the system so a sufficient number of individual features are measured so a good statistically based, scatterometry analysis can be performed.
The assignee of the subject invention has previously developed simultaneous multiple angle of incidence measurement tools which have been used to derive characteristics of thin films on semiconductor wafers. It is believed that data from the same type of tools can be used with an appropriate scattering model analysis to determine critical dimensions and feature profiles on semiconductors.
Detailed descriptions of assignee""s simultaneous multiple angle of incidence devices can be found in the following U.S. Pat. Nos.: 4,999,014; 5,042,951; 5,181,080; 5,412,473 and 5,596,411, all incorporated herein by reference. The assignee manufactures a commercial device, the Opti-Probe which takes advantage of some of these simultaneous, multiple angle of incidence systems. A summary of all of the metrology devices found in the Opti-Probe can be found in PCT application WO/9902970, published Jan. 21, 1999.
One of these simultaneous multiple angle of incidence tools is marketed by the assignee under the name beam profile reflectometer (BPR). In this tool, a probe beam is focused with a strong lens so that the rays within the probe beam strike the sample at multiple angles of incidence. The reflected beam is directed to an array photodetector. The intensity of the reflected beam as a function of radial position within the beam is measured and includes not only the specularly reflected light but also the light that has been scattered into that detection angle from all of the incident angles as well. Thus, the radial positions of the rays in the beam illuminating the detector correspond to different angles of incidence on the sample plus the integrated scattering from all of the angles of incidence contained in the incident beam. In this manner, simultaneous multiple angle of incidence reflectometry can be performed.
Another tool used by the assignee is known as beam profile ellipsometry. In one embodiment as shown and described in U.S. Pat. No. 5,042,951, the arrangement is similar to that described for BPR except that additional polarizers and/or analyzers are provided. In this arrangement, the change in polarization state of the various rays within the probe beam are monitored as a function of angle of incidence.
It is believed that the data generated by either of these tools could be used to appropriately model and analyze critical dimensions and feature profiles on semiconductors.
The lens used to create the probe beam spot from a laser source in the above two simultaneous multiple angle of incidence systems is typically larger than the distance between adjacent features of the periodic structure of interest. However, in order to provide statistically significant information, it is desirable that information be collected from at least twenty or more of the repeating features. One method of achieving this goal is to increase the spot size of the probe beam. Such an approach is described in U.S. Pat. No. 5,889,593 incorporated by reference. In this patent, a proposal is made to include an optical imaging array for breaking up the coherent light bundles to create a larger spot.
It is believed the latter approach is not desirable because of the additional complexity it introduces into the measurement. Ideally, when attempting to analyze a periodic structure (e.g., a periodic critical dimension array) it is desirable to have no additional periodicities in the measurement system between the source and detector. Multiple periodic signals are more difficult to analyze and are often plagued with added uncertainty and ambiguity with respect to extracting parameters associated with any of the constituent components.
In accordance with the subject invention, the requirement for increasing the area over which measurements are taken is achieved in two different ways. In the first approach, the probe beam spot is scanned over the wafer until a sufficient amount of data are taken. Once the data are taken, a spatial averaging algorithm is utilized. Spatial averaging is discussed in U.S. patent application, Ser. No. 09/658,812, filed Sep. 11, 2000 and incorporated herein by reference. In another approach, the probe beam is generated by an incoherent or white light source. When incoherent light is focused by a lens, the spot size will be significantly larger than with a laser. No separate imaging array needs to be included to break up the coherence of the light as in the prior art. In such a system, a monochrometer could be located between the light source and the detector to permit measurement of a narrow band of wavelengths. The wavelength selected can be matched to the type of sample being inspected in order to obtain the most statistically relevant data. In addition, it would also be possible to scan the monochrometer in order to capture data at multiple wavelengths. It would also be possible to measure multiple wavelengths simultaneously as described in U.S. Pat. No. 5,412,473.
Alternatively or in addition, the measurement data which can be obtained from two or more metrology devices of the type described in the above identified PCT application, could be used to advance this analysis. As more of these metrology devices are added, the ability to unambiguously distinguish features increases. Thus, it is within the scope of the subject invention to utilize either or both of a simultaneous multiple angle of incidence spectrometer or ellipsometer along with one or more of spectroscopic reflectometry, spectroscopic ellipsometry or absolute ellipsometry tools with the latter two being deployed in a manner that maximizes the information content of the measurement. For example, with a rotating compensator spectroscopic ellipsometer one measures both the sign and magnitude of the ellipsometric phase while in more standard configurations, e.g., a rotating polarizer/rotating analyzer, only the magnitude or phase can be measured.
An example of an analytical approach for evaluating critical dimensions using data from a broadband reflectometer is described in xe2x80x9cIn-situ Metrology for Deep Ultraviolet Lithography Process Control,xe2x80x9d Jakatdar et. al. SPIE Vol. 3332, pp. 262-270 1998. An example of using a spectroscopic ellipsometer equipment for CD metrology is described in, xe2x80x9cSpecular Spectroscopic Scatterometry in DUV Lithography, SPIE Vol. 3677, pp 159-168, from the SPIE Conference on Metrology, Inspection and Process Control for Microlithography XIII, Santa Clara, Calif., March 1999.
Further and related information measuring critical dimensions can be found in U.S. Pat. No. 5,830,611 and 5,867,276, incorporated herein by reference.